Extruders and injection molding machines use a screw to feed material through the machines. The screw has a flight and a channel. In addition, the screw is located inside a barrel. As the screw rotates inside the barrel, the flight advances the material through the barrel. For plastic material, the screw flight is designed to cause the material to melt as it advances through a heated barrel. In addition, the screw may be used to mix two or more materials as they are moved through the barrel. Furthermore, the machine may use several screws.
The machine is designed and manufactured with a predetermined distance between the screw flight and the barrel. During use, the movement of material through the screw and barrel causes wear which increases the distance between the screw flight and the barrel. If the material is abrasive, the wear occurs more quickly.
Generally an increase in wear creates a reduction of the shear rate on the polymer, resulting in less mixing to occur in the barrel. This results in molten polymer with poor homogeneity. A non-homogeneous product causes the risk of fabricating plastic parts which will fail to meet performance criteria established on the physical properties. Preventive maintenance procedures to monitor screw and barrel wear presently used by the industry require machines to be shut down, disassembled, and physically measured to determine actual wear. This process is time consuming, and generally requires one-half to one full day of lost productivity and the use of high cost maintenance labor, causing the process to be expensive.
A method is needed which can be more efficient and more economical for the determination of screw wear.